The present invention relates generally to liquid level detection systems and, more particularly, to an internal liquid level detector system for mounting within a vessel for measuring the volume of liquid within the vessel and a method for operating such a system.
Conventional storage vessels, such as propane tanks, are beneficial both for their mobility and for their suitability to the energy needs of remote locations where access to other forms of energy is often unavailable or costly. It is desirable to be able to detect the volume of liquid in such a vessel at any specific time so as to schedule re-supply (sometimes referred to as re-charging) of the tank thereby to prevent emptying of the vessel (sometimes referred to as xe2x80x9crunning dryxe2x80x9d).
Sensors for measuring the elevation of the upper surface of liquid contained in a vessel are known. In operation, such a sensor generates an electrical signal indicative of the quantity of liquid in the vessel. The signal is transmitted to an interface located externally of the vessel which communicates to a user the quantity of liquid in the vessel.
One such type of internal sensor is a float-type gauge. Such a gauge normally has limited accuracy because it contains moving parts such as the float and associated linkage for sensing vertical displacement of the float as the upper surface of the liquid rises and falls. With movement comes associated friction to which the moving parts are subjected thereby causing wear to the gauge and eventual degradation. Additionally, a float-type gauge includes fragile portions, such as the linkage connected to the float which must be sufficiently delicate to translate vertical displacement of the float to the stationary base of the gauge for measuring the displacement. The fragility of the moving parts makes them susceptible to damage.
Alternatively, a computer may be used to estimate the level, i.e., elevation, of the liquid in a vessel relative to the interior bottom surface of the vessel. In particular, specially designed software may be used to monitor usage and predict when refill or recharging is necessary. Previous liquid usage patterns are considered, as well as weather and other data. While of some use, computers have been found to have limited reliability in predicting the amount of liquid in the vessel at a specific time and, in particular, the time when the vessel will become completely empty if not recharged. Such prediction may be especially difficult when the usage patterns are variable.
The internal liquid level detector system of the present invention detects the level of a liquid in a vessel. The system comprises a detector assembly including a thermally conductive substrate supported within the vessel. One or more heaters (referred to herein as xe2x80x9cheater(s)xe2x80x9d) are mounted on the substrate within the vessel such that the heater(s) are thermally coupled to the interior of the vessel. The heater(s) may be actuated to add heat to the surface of the substrate thermally coupled to the interior of the vessel. If multiple heaters are mounted on the substrate, the heaters are located thereon such that, when the substrate is supported in the vessel, the heaters have different elevations.
One or more sensors (referred to herein as xe2x80x9csensor(s)xe2x80x9d) are mounted on the substrate in proximity to the heater(s). The sensor(s) are thermally coupled to the interior of the vessel to detect the temperature(s) therein in proximity to the sensor(s). The sensor(s) may be actuated to generate an electrical signal defining a temperature signal(s) indicative of the detected temperature(s). If multiple sensors are mounted on the substrate, the sensor(s) are located thereon such that, when the substrate is supported in the vessel, the elevation of each sensor corresponds to the elevation of a respective one of the heaters.
A processor is electrically connected to the sensor(s) for receiving the temperature signal after actuation of the associated heater(s). The processor is programmed to calculate a xe2x80x9ctemperature indexxe2x80x9d proportional to the temperature signal(s). The temperature index(s) indicates the temperature increase(s) detected by the sensor(s) resulting from actuation of the heater(s). This, in turn, indicates the presence of liquid or vapor adjacent to the sensor(s) because the presence of liquid results in a lower temperature increase as compared to the temperature increase if vapor is present.
The processor is programmed further to compare the temperature index(s) to a xe2x80x9creferencexe2x80x9d and to generate an electrical signal(s) defining an elevation signal(s) indicative of the elevation of the upper surface of the liquid being either above or below the sensor(s).
The processor is electrically connected to an interface for communicating to a user the elevation of the upper surface of the liquid in the vessel. The aforementioned system is operated according to a method for detecting the level of liquid in the vessel.
The system of the present invention provides for the heater(s) and sensor(s) to be within the vessel. Accordingly, the heat transfer between the heater(s), liquid and/or vapor within the vessel, and sensor(s) need not be through the exterior wall of the vessel. Heat transfer through the wall of the vessel is typically of limited efficiency because of the substantial thickness of the vessel normally required if the contents of the tank are pressurized. In contrast, if the substrate of the present invention is a tube, such a tube may have a considerably smaller outer diameter (e.g., less than xc2xcxe2x80x3 to xc2xexe2x80x3) and can be made with thin walls because it is inherently stronger due to its geometry. This results in a reduction in the required output of the heater allowing for a reduction in the amount of energy required resulting in a lower wattage.
Moreover, providing the heater(s) and sensor(s) within the vessel provides a more intimate thermal coupling between the liquid and vapor in the vessel, and the heater(s) and sensor(s) thereby increasing the accuracy of the system. The thermal coupling is facilitated by the heater(s) and sensor(s) being surrounded by the liquid and vapor in the vessel. In contrast, locating the heater and sensor on the outer surface of the vessel results in portions of the heater and sensor facing away from the liquid and vapor in the vessel thereby proving possible paths for diversion of the heat transfer between the heater and sensor, and the liquid and vapor. Moreover, the substrate may be a tube having a relatively small outer diameter and thin wall thickness which improves the thermal coupling with the interior contents of the vessel.
Additionally, locating the heater(s) and sensor(s) on the outer surface of the vessel requires the heat transfer between such heater(s) and sensor(s), and the liquid and vapor in the vessel, to be through the relatively thick wall of the vessel. The increased thickness of the wall of the vessel is typically needed because the vessel must have sufficient strength to contain the contents within the vessel which are often under high pressure. The increased thickness of the vessel wall normally complicates the temperature detections by the sensor and the resultant determinations by the processor because the heat transfer through the thicker wall of the vessel is more complex. This complexity results, e.g., from a portion of the energy of the heater (located outside the vessel) being absorbed and distributed by the vessel wall rather than by the contents of the vessel. This requires the processor to interpret a reduced and less distinct signal from the sensor.
Furthermore, providing the heater(s) and sensor(s) within the vessel enables the substrate, e.g., a tube, to be a linear structure. In contrast, if the substrate is mounted on the outer surface of the vessel, which frequently is curved, the substrate is preferably shaped to conform to the curvature of the outer surface to enable close adjacency between the heater and sensor and the vessel. A curved substrate matching the curvature of the outer surface of the vessel introduces further complexity into the structure of the substrate. This curvature may be substantial if the components are separated by substantial distances. Such separation, especially vertically, may be necessary for the system to provide detection of a substantial range of liquid levels. While this may appear to be remedied by dispensing with the substrate entirely (and mounting the heater and sensor directly to the vessel), the substrate is desirable for a heater and sensor attached to the outer surface of the vessel because the substrate advantageously provides modularity. Such modularity facilitates handling of the heater and sensor by a single structure and provides protection to the components.
The system of the present invention does not encompass any moving parts within the vessel thereby eliminating any associated mechanical friction and ensuring durability, longevity and ruggedness. The absence of moving parts also increases accuracy and reproducibility. Additionally, the heater(s) and sensor(s) may be contained within a non-deformable enclosure, such as a tube, which provides a complete barrier to direct contact between the heater(s) and sensor(s) and the environment within the vessel. Such an enclosure may be contained within a perforated, non-deformable second enclosure, such as a second tube, for protective shielding from the environment within the vessel.
The substrate of the present invention provides for the heater(s) and sensor(s) to be included as multiple components within a single modular component. This facilitates installation in a tank, especially if the tank is buried, because only a single assembly is required to be inserted, e.g., from above by lowering into the tank. This modularity also facilitates the durability and reliability of the system.
These and other features and advantages of the invention will be more fully understood from the following description of specific embodiments of the invention taken together with the accompanying drawings.